1. Field of the Invention
The present invention relates to a heat-sensitive flow rate sensor for use in measuring flow rate and flow velocity of a fluid and, more particularly, to a heat-sensitive flow rate sensor of the type which measures flow velocity and flow rate of a fluid based on the rate at which heat is carried away from a probe by the fluid which flows in contact with the probe.
2. Description of the Related Art
FIG. 5 schematically shows the construction of a known heat-sensitive flow rate sensor of the type which is disclosed in Japanese Utility Model Laid-Open No. 61-108930. A sensor tube 2, which forms a part of the fluid passage, is provided at a predetermined position in a housing 1 which defines a principal passage for the fluid. A measurement resistor unit 3 including a heat-sensitive resistor 6 (see FIGS. 6A, 6B), as well as a fluid temperature sensor 4, is disposed at predetermined location in the sensor tube 2. The measuring resistor unit 3 and the fluid temperature sensor 4, together with resistors R1 and R2, form a bridge circuit. The junctions b and c of the bridge circuit are connected to a differential amplifier 101. The output of the differential amplifier 101 is connected to the base of a transistor 102. The transistor 102 is connected at its emitter to a junction a of the bridge circuit and at its collector to a power supply 103.
FIGS. 6A and 6B are a front elevational view and a side elevational sectional view of an example of the measuring resistor unit 3 of the heat-sensitive flow rate sensor. Referring to these Figures, the measuring resistor unit 3 has a substrate 5 made of an insulating material such as alumina on which is formed a heat-sensitive resistor 6 in the form of a film. The heat-sensitive resistor 6 is made of a material which varies its resistivity according to temperature, and more specifically, a material having a positive temperature coefficient. Trimming line wirings 7 are laid on the heat-sensitive resistor 6 so as to provide paths of electrical currents. Lead lines 8 are connected to an end of the resistor 6. A protective coat 9 is formed on the heat-sensitive resistor 6 so as to protect the latter. The measuring resistor unit 3 is supported in the detecting tube 2 by a support portion 10.
The operation of this known heat-sensitive flow rate sensor is as follows. When flow of a fluid at a constant flow rate exists in the housing 1, the bridge circuit is balanced in such a manner that the mean temperature of the heat-sensitive resistor 6 of the measuring resistor unit 3 is maintained at a level which is higher than the fluid temperature by a predetermined value, by the control of the electrical current supply to the bridge circuit. The control of the electrical current supply is performed by a control circuit constituted by the differential amplifier 101 and the transistor 102. When the flow rate of the fluid is changed, rates of heat conduction to the surfaces of the heat-sensitive resistor 6 and the supporting substrate 5 are changed. This change varies the temperature of the measuring resistor unit 3, causing a corresponding change in the resistivity of the measuring resistor unit 3, so that an imbalance is caused in the bridge circuit. The control circuit then operates to increase the electrical current supplied to the bridge circuit. Consequently, the heat-sensitive resistor 6 is heated so that the mean temperature of the resistor 6 is elevated to the level exhibited before the change in the fluid flow rate, whereby the bridge circuit is balanced again. The level of the electrical current supplied to the measuring resistor unit 3 is used to measure the flow rate of the fluid. The fluid temperature sensor 4, which is held by another supporting substrate and which is made of a resistor having temperature-dependency of resistivity, provides compensation for change in the output which otherwise is caused by a change in the fluid temperature.
In the known heat-sensitive flow rate sensor having the described construction, not only the heat transferred to the fluid to be measured but also the heat conducted from the supporting substrate 5 to the supporting portion 10 of the measuring resistor unit 3 is sensed. The supporting substrate 5 and the supporting portion 10 which supports the measuring resistor unit 3 have large heat capacities as compared with the heat-sensitive resistor 6. Such undesirable movement of heat inconveniently prolongs the time until the expected operating temperature is reached in the heat-sensitive flow rate sensor, while retarding response of the flow rate sensor to change in the flow rate of the fluid. Consequently, matching or re-establishment of the steady temperature distribution is completed only after elapse of a longtime.
This is attributable to facts that large temperature gradients are formed in the thermal transition regions between the heated portion and non-heated portion of the supporting substrate 5 of the heat-sensitive resistor 6 and between the supporting substrate 5 and the supporting portion 10 of the measuring resistor unit 3. Consequently, the level of the electrical energy supplied to the heat-sensitive resistor 6 does not accurately indicate the flow rate of the fluid, in the transient period before the required operating temperature is reached by the heat-sensitive flow rate sensor. Thus, the known heat-sensitive flow rate sensor involves a risk of inaccurate measurement of the flow rate.